The probe received a low-frequency radio signal from the atmosphere of Venus

The probe received a low-frequency radio signal from the atmosphere of Venus
The probe received a low-frequency radio signal from the atmosphere of Venus

During a close flyby of Venus in July 2020, NASA's Parker Solar Probe discovered something strange.

Descending 833 kilometers above the surface of Venus, the probe's instruments recorded a low-frequency radio signal - a sure sign that Parker was gliding through the ionosphere, a layer of the planet's upper atmosphere.

This is the first time that the instrument has been able to record direct measurements of Venus's upper atmosphere in situ in nearly three decades, which show how Venus changes in response to cyclical changes in the Sun.

“I was so excited to get new data from Venus,” said astronomer Glyn Collinson of NASA's Goddard Space Flight Center.

For us here on Earth, Venus is a delightful world. It is so similar to our planet in size and composition, but so different: a poisonous, incandescent world, completely inhospitable for life as we know it.

However, there were relatively few missions to explore Venus. It doesn't make much sense to send landing pages; they will not be able to operate on the planet's surface at 462 degrees Celsius (864 degrees Fahrenheit).

Sending orbiting probes is also considered problematic due to the incredibly dense atmosphere of carbon dioxide and rain clouds of sulfuric acid that make it difficult to determine what is happening on the surface.

For these reasons, Venus has not been a popular target for special missions for some time (a recent exception is the Japanese orbiter Akatsuki), and much of the data comes in chunks from instruments with other primary targets, such as the Parker Solar Probe.

As Parker is on his mission to study the Sun in detail, he is using Venus for gravity assist. It was during one of these overflights that a radio signal was recorded from the probe's instruments.

Collinson, who has worked on other planetary missions, noticed a strange pattern that he could not pinpoint in the waveform.

“The next day I woke up,” he said. "And I thought," My God, I know what this is!"

It was the same type of signal recorded by the Galileo probe as it slid through the ionospheres of Jupiter's moons - a layer of the atmosphere also observed on Earth and Mars, where solar radiation ionizes atoms, resulting in a charged plasma that produces low-level plasma.

Once the researchers figured out what the signal was, they were able to use it to calculate the density of the Venusian ionosphere and compare it with the last direct measurements made back in 1992. Surprisingly, the ionosphere was an order of magnitude thinner than in 1992.

The team believes it has something to do with solar cycles. Every 11 years the poles of the Sun change places; south becomes north and north becomes south. It's unclear what drives these cycles, but we do know that the poles switch when the magnetic field is weakest.

Since the sun's magnetic field controls its activity - such as sunspots (temporary regions of strong magnetic fields), solar flares and coronal mass ejections (caused by the closure and reunification of magnetic field lines) - this stage of the cycle manifests itself in minimal activity. This is called the solar minimum.

As soon as the poles are swapped, the magnetic field increases and solar activity rises to a maximum before subside again for the next polar reversal.

Measurements of Venus from Earth showed that Venus's ionosphere changed in sync with solar cycles, becoming thicker at the maximum and thinner at the minimum. But it was difficult to confirm this without direct measurements.

Measurement 1992was carried out in a period close to the solar maximum; the 2020 measurement is close to the solar minimum.

“When multiple missions, one after another, confirm the same result, it gives you more confidence that thinning of the atmosphere does exist,” said astronomer Robin Ramstad of the University of Colorado at Boulder.

Maybe it's time for another mission to Venus?

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